Shaker is a voltage-gated potassium ion channel and a model system for understanding the structure-function principles underlying all voltage-gated ion channels. These channels underlie excitation propagation in nerves, and channel mutations cause various cardiac, neuronal, and neuromuscular diseases. It is known that these ion channels are turned on and off (i.e. change their conductivity to ion flow) by changes in voltage across the membrane. But how is this achieved? Specifically, one part of the ion channel is known to be the "voltage-sensor," but how this moves in order to gate the channel on and off is not known. Recently, crystallographic data (of the KvAP channel) has led to a new and very different model of voltage-gating which is highly controversial and seems incongruous with biophysical and biochemical data. [unreadable] [unreadable] We are applying a technique called Luminescence Resonance Energy Transfer (LRET) to answer the biggest question in the field: Is the proposed KvAP model accurate for functional channels in a membrane? LRET is capable of measuring distances and distance changes between two sites on a protein with subangstrom precision. We have shown that LRET signals on the Shaker voltage-sensor strongly correlate with electrophysiological measurements [1]. Now we have developed a new configuration for LRET that measures the distance from sites on the voltage-sensor to a scorpion toxin bound to the external mouth of the ion pore. With this arrangement we can test rigorously whether the voltage-sensor has a large transmembrane movement, as proposed in the KvAP model. We will use LRET and conventional FRET to define more exactly the conformational changes that underlie channel opening and closing. By extending LRET to other sites not previously tested, we will greatly constrain models of the voltage-sensor structure, which will assist in interpreting the recent crystallographic data. We also are using LRET to study the voltage-sensor of a mutant Shaker called ILT, which allows us to measure separately conformational changes associated with several steps along the multi-step channel opening process. Potential developments in the design and synthesis of new luminescent probes, making the chelates more suitable as LRET donors (and for other studies), are presented. [unreadable] [unreadable]